1. Field of the Invention
This invention relates to a process for making mixtures of olefin polymers and copolymers in a single reactor by in-situ contact of one or more olefinic compounds with a mixture of two catalysts comprising a supported hexavalent chromium catalyst, and a supported beta-stabilized, tetrahydrocarbyl zirconium.
2. Brief Description of the Prior Art
In U.S. Pat. No. 4,806,513, issued to McDaniel et al a process is disclosed in which olefin is polymer binary catalyst system to yield a mixture of polymers having a bimodal molecular weight distribution, and which exhibits good environmental stress crack resistance and high stiffness. The polymers are produced concurrently in a single reactor. The dual or binary catalyst system includes chromium on silicated alumina, which is further fluorided and/or phosphated. Those catalysts can be used in combination with an organochromium compound on aluminophosphate, or in combination with a zirconium catalyst system. In one of the binary catalyst systems, hexavalent chromium/aluminum phosphate is used as one of the catalysts, and chromium on silicated fluorided alumina is used as the other. This combination of catalysts, when used to polymerize olefins, yields a blend of polymers having a bimodal molecular weight distribution which exhibits good environmental stress crack resistance and good stiffness.
U.S. Pat. No. 4,056,669 teaches the use of tetrabenzyl zirconium supported on alumina as a polymerization catalyst for polymerizing olefins.
It is also known to use phosphated alumina as a hexavalent chromium catalyst support, and such is disclosed in U.S. Pat. No. 4,397,765. Trihydrocarbyl boron compounds can be utilized as cocatalysts. This patent also discloses the production of polyolefins of high melt index, and improved density and stress crack resistance when a high P/Al atomic ratio is characteristic of an aluminum phosphate support material.
A catalyst system used for olefin polymerization is also disclosed in U.S. Pat. No. 4,364,842. In this catalyst system, hexavalent chromium on an aluminum phosphate support is the catalyst employed, and it is utilized with a cocatalyst which is a boron compound selected from trialkyl boron compounds, boron alkoxides and mixtures thereof.
The disclosure of U.S. Pat. No. 4,364,841 is similar to that of U.S. Pat. No. 4,364,842, but in this patent the supported chromium is in the xerovalent state.
It is also known from the prior art that certain advantages are derived from the blending of a high molecular weight olefinic polymer with a low molecular weight olefinic polymer. The polymers are separately produced with a Ziegler catalyst system, and are then physically blended after production. The advantages of such blending, the procedures for blending and the properties of such blends are described in U.S. Pat. No. 4,461,873.
Another dual catalyst mixture is described in U.S. Pat. No. 4,285,834. In this system, at least two independently supported catalysts are employed in a single reactor. At least one of these is an inorganic halide supported on titanium and/or vanadium Ziegler-type catalyst. The other of the two catalysts which is included in this system is a hexavalent chromium compound supported on an inorganic oxide. This catalyst system is particularly suited for polymerizing one or more .alpha.-olefin compounds. In the process described in U.S. Pat. No. 4,285,834, the independently supported catalysts are mixed either just prior to introduction into the reaction zone, or they may be added separately and concurrently to the polymerization vessel containing the olefinic material to be polymerized. In this system, polymerization is preferably carried out under solution polymerization conditions.